1. Field of the Invention
The present invention relates to boundary acoustic wave devices for use in, for example, resonators and filters, and particularly, to a boundary acoustic wave device having a three-medium structure in which a first medium layer and a second medium layer are arranged on a piezoelectric substrate made of LiTaO3.
2. Description of the Related Art
In recent years, boundary acoustic wave devices have been substituted for surface acoustic wave devices and have been attracting much attention. The boundary acoustic wave devices do not require packages having cavities. Therefore, the use of the boundary acoustic wave devices is effective to achieve compact resonators and filters.
WO 98/52279 discloses an exemplary boundary acoustic wave device having a three-medium structure in which a first medium layer and a second medium layer are arranged on a piezoelectric substrate. With reference to FIG. 31, interdigital electrodes (not shown) are arranged on a piezoelectric substrate 1001 and a polycrystalline silicon oxide layer defining a first medium layer 1002 and a polycrystalline silicon layer defining a second medium layer 1003 are arranged on the piezoelectric substrate 1001. The IDT electrodes are arranged at the interface between the piezoelectric substrate and the polycrystalline silicon oxide layer.
The presence of the polycrystalline silicon layer allows the energy of a boundary acoustic wave excited by the IDT electrodes to be confined in the first medium layer 1002, which is made of the polycrystalline silicon oxide layer, as shown in FIG. 31.
Even if the quality of the polycrystalline silicon layer is deteriorated, electrical properties of the boundary acoustic wave device are unlikely to be deteriorated. Since the polycrystalline silicon oxide layer and the polycrystalline silicon layer protect the IDT electrodes, the reliability of the boundary acoustic wave device is outstanding. The use of the three-medium structure is particularly effective for higher frequencies.
WO 98/52279 does not address changes in properties depending on the thickness of the IDT electrodes. However, it has been determined that when IDT electrodes have an increased thickness or are made of a high-density metal, an undesired transverse-mode ripple is excited in such a conventional boundary acoustic wave device as disclosed in WO 98/52279. The three-medium structure, which includes LiTaO3 as disclosed in WO 98/52279, is not always capable of obtaining good properties.